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-40 & master cylinder freezing

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Car is a 98 legacy outback in Fairbanks AK

I have been getting squishy brakes around -20F and a very hard pedal around -40F/C. I flushed all the brake lines last weekend and this only slightly helped. I did notice recently that even at -40F and letting the car warm up for 30 minutes in the parking lot that the brakes worked fine, once I got on the highway though and tried the brakes on the exit ramp the pedal was hard.

I believe that when the master cylinder is cold past -30 or the problem occurs. So I think I will try and find a way to keep the cold out from under the hood, cardboard on radiator. I would also like to plug up the fake hood air intake better than the stock black plastic thing does.

Any ideas or similar problems?
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Car: 2008 Tribeca, 2010 LGT, Sold: 2005 XT Limited
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Sounds like lost of brake booster because of something freezing. That's where I would be looking. Of course cardboard at the radiator is a good idea any which way. Better fuel economy give those temperatures!
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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are there 'special' brake fluids you guys use? Maybe investigate that.

it does sorta seem like the booster is re-freezing when the car is moving down the road and the warm air in the engine bay is flushed out with arctic air.

maybe some kinda insulation around the booster line or even a light bulb placed near it. Wire it to come on when the heater is turned on or ???
 

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Silicone-based brake fluid (DOT 5) is indicated for extreme cold... but usually only for non-ABS cars that have never had glycol-based (DOT 3 or DOT 4) fluids in them. I don't know about changing over a car that has already had glycol-based fluid used in it.

I grew up in Fairbanks and I don't recall that we ever had this specific problem on the family cars. Block heaters, battery blankets, grille covers, spare fuses for the power outlets on municipal parking meters, "storm window" appliques for the side windows and some other stuff too... but I don't recall anything to prevent frozen brake fluid.
 
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